This optical micrograph shows a cross section of fingerlike projections, known as villi, from the uppermost part of a mammalian intestine. The inner surface of the intestine renews itself every three to five days as epithelial cells migrate from the bottom to the top of the villi, where they are shed into the intestine and discarded. For more on collective cell migration in living tissue, turn to the article by Ricard Alert and Xavier Trepat on page 30 . (Image by Steve Gschmeissner/Science Photo Library.)